Smoke and Mirrors: Shadowing Files at a Geographically Remote Location Without Loss of Performance August 2008 Hakim Weatherspoon, Lakshmi Ganesh, Tudor Marian, Mahesh Balakrishnan, and Ken Birman Large-Scale Distributed Systems and Middleware (LADIS) September 15 th, 2008

Critical Infrastructure Protection and Compliance  U.S. Department of Treasury Study Financial Sector vulnerable to significant data loss in disaster Need new technical options  Risks are real, technology available, Why is problem not solved?

Bold New (Distributed Systems) World…  High performance applications coordinate over vast distances Systems coordinate to cache, mirror, tolerate disaster, etc. Enabled by cheap storage and cpus, commodity datacenters, and plentiful bandwidth  Scale is amazing, but tradeoffs are old ones

Mirroring and speed of light dilemma…  Want asynchronous performance to local data center  And want synchronous guarantee Primary site Remote mirror async sync Conundrum: there is no middle ground

Mirroring and speed of light dilemma…  Want asynchronous performance to local data center  And want synchronous guarantee Primary site Remote mirror sync Conundrum: there is no middle ground Local-sync Remote-sync

Challenge  How can we increase reliability of local-sync protocols? Given many enterprises use local-sync mirroring anyways  Different levels of local-sync reliability Send update to mirror immediately Delay sending update to mirror – deduplication reduces BW

Talk Outline  Introduction  Enterprise Continuity How data loss occurs How we prevent it Smoke and mirrors file system  Evaluation  Conclusion

How does loss occur? Primary site Remote mirror  Rather, where do failures occur?  Rolling disasters Packet loss Partition Site Failure Power Outage

Enterprise Continuity: Network-sync Local-sync Network-syncRemote-sync Wide-area network Primary site Remote mirror

Enterprise Continuity Middle Ground  Use network level redundancy and exposure reduces probability data lost due to network failure Primary site Remote mirror Data Packet Repair Packet Network-level Ack Storage-level Ack

Enterprise Continuity Middle Ground  Network-sync increases data reliability reduces data loss failure modes, can prevent data loss if At the same time primary site fail network drops packet And ensure data not lost in send buffers and local queues  Data loss can still occur Split second(s) before/after primary site fails… Network partitions Disk controller fails at mirror Power outage at mirror  Existing mirroring solutions can use network-sync

Smoke and Mirrors File System  A file system constructed over network-sync Transparently mirrors files over wide-area Embraces concept: file is in transit (in the WAN link) but with enough recovery data to ensure that loss rates are as low as for the remote disk case! Group mirroring consistency

Mirroring consistency and Log-Structured File System B2B1 append( B1,B2 ) V1R1I2B4B3I1 append( V1.. ) V1 R1 I2 I1 B2 B1 B3B4

Talk Outline  Introduction  Enterprise Continuity  Evaluation  Conclusion

Evaluation  Demonstrate SMFS performance over Maelstrom In the event of disaster, how much data is lost? What is system and app throughput as link loss increases? How much are the primary and mirror sites allowed to diverge?  Emulab setup 1 Gbps, 25ms to 100ms link connects two data centers Eight primary and eight mirror storage nodes 64 testers submit 512kB appends to separate logs  Each tester submits only one append at a time

Data loss as a result of disaster  Local-sync unable to recover data dropped by network  Local-sync+FEC lost data not in transit  Network-sync did not lose any data Represents a new tradeoff in design space Primary site Remote mirror - 50 ms one-way latency - FEC(r,c) = (8,3) Local- sync Network- sync Remote- sync

High throughput at high latencies

Application Throughput  App throughput measures application perceived performance  Network and Local-sync+FEC tput significantly greater than Remote-sync(+FEC)

…There is a tradeoff

Latency Distributions

Conclusion  Technology response to critical infrastructure needs  When does the filesystem return to the application? Fast — return after sending to mirror Safe — return after ACK from mirror  SMFS — return to user after sending enough FEC  Network-sync: Lossy Network  Lossless Network  Disk!  Result: Fast, Safe Mirroring independent of link length!

 Questions?